1. Field of the Invention
The present invention relates to a zoom lens system used for a small-size taking optical system, and more specifically, to a compact and high-magnification zoom lens system being suitable for a taking optical system for a digital input/output apparatus such as a digital still camera and a digital video camera.
2. Description of the Related Art
In recent years, with the spread of personal computers, digital still cameras and digital video cameras (hereinafter, referred to as digital cameras) capable of easily capturing image information into digital apparatuses have been spreading among personal users. It is expected that such digital cameras will increasingly spread as image information inputting apparatuses.
Generally, the image quality of a digital camera depends on the number of pixels of the solid-state image sensing device such as a CCD (charge coupled device). At present, the solid-state image sensing device mainly used for consumer digital cameras is a so-called VGA-class solid-state image sensing device having approximately 330 thousand pixels. However, the image quality of the VGA-class camera largely pales in comparison with the image quality of conventional cameras using silver halide film. For this reason, consumer digital cameras have recently been desired to have high image quality exceeding one million pixels, and the taking optical systems of these digital cameras have also been desired to satisfy high image quality.
Since these consumer digital cameras have also been desired to perform zooming, particularly, optical zooming in which image quality degradation is small, digital camera zoom lens systems having high magnification and satisfying high image quality have been required in recent years.
However, conventional digital camera zoom lens systems satisfying the high image quality exceeding one million pixels are mostly interchangeable lens systems for single-lens reflex cameras that are diverted to digital camera taking optical systems, and zoom lens systems for large-size digital cameras for business use. These zoom lens systems are unsuitable for consumer digital cameras because they are large in size and high in cost.
On the other hand, it is considered to divert, to digital camera taking optical systems, taking optical systems for silver halide film lens shutter cameras that have been remarkably reduced in size and increased in magnification in recent years.
However, when a taking optical system for a lens shutter camera is used in a digital camera as it is, it is impossible to sufficiently satisfy the light condensing performance of a micro lens provided on the front surface of the solid-state image sensing device of the digital camera, so that the image brightness extremely differs between the central portion and the peripheral portion of the image. This problem is caused because the off-axial luminous flux exiting from the lens shutter camera taking optical system is obliquely incident on the image plane since the exit pupil of the taking optical system is situated near the image plane. If the distance between the image plane and the exit pupil position of the conventional lens shutter camera taking optical system is increased to solve this problem, the taking optical system will inevitably increase in overall size.